A common type of lifting body consists of a closed, rigid tank which is filled to a controllable amount with a lifting gas. Such a lifting body has a fixed volume which is not variable if the lifting body is placed at different depths. The invention relates, however, to lifting bodies in which the lifting gas assumes approximately the same pressure as the surrounding mass of water. Examples of such lifting bodies are balloons and also rigid lifting bodies in which the enclosed mass of water is in uninterrupted communication with the mass of water lying outside.
Lifting bodies of the type described above are known and used for recovering heavy objects from the sea floor for example. Previous attempts to make them vertically controllable by regulating the gas supply depending on the depth has met with little success.
There is the particular problem that such a lifting body is difficult to depth stabilize. A comparison might be made with the swimming bladder of a fish, which provides an unstable depth equilibrium, because a deviation upwards involves a reduction in pressure and an increase in volume, thus increasing the lifting force, and the reverse is true of a deviation downwards. Therefore, it has been a common understanding by persons skilled in the art that a lifting body of the type described above cannot be depth stabilized. We will now demonstrate that this understanding is in principle correct.
The block diagram in FIG. 1 illustrates a control system, in which a balloon 1 obtains air via a controlled valve 6. A position measuring instrument 2 (pressure meter or echo sounder) provides a position signal which is compared in a comparator 3 with a desired position signal, and the result is set to a regulator 4, which controls, via a transformer 5, the valve 6 for controlling the supply or removal of the air.
We assume that the regulator 4 is a proportional regulator.
Assume now that the balloon is at a certain depth X which agrees with the set desired position. We now change the set point to a higher desired level. The system then opens the valve 6, and more air is pressed into the balloon. The lifting force will increase in proportion to the added air (at least as long as the balloon does not rise apreciably). The increased amount of air causes the balloon to accelerate upwards, and the velocity increases as it moves, and the new set desired position will be reached. When the new desired position has been reached, the balloon, however, has been accelerated to a velocity so that the balloon passes the desired position. At that position, the valve 6 will of course cut off the supply of air and open the air release. The acceleration will, however, continue until the previously added air has been released, and only when the negative regulator error has become significantly greater than the positive starting position, will the acceleration upwards cease. The velocity is still directed upwards, however, and it is obvious that the balloon will continue further before it turns. The release of air and the movement downwards will proceed in a similar manner, and the balloon's next lower turning point will lie lower than the starting level. In this manner the oscillating amplitude of the balloon will become greater and greater.
The control equation for such a system can be written: ##EQU1## in which y is the measured depth and X is the set value. Such a control system is instable.
Some improvement can be achieved if instead of a valve opening corresponding to the difference between the set and actual values (X-y), one works with a system which begins to release air even before the correct level has been passed (proportional and derivative regulator), but it is still not possible to make the system stable. Furthermore, the above reasoning has been simplified in a manner favoring stability, in that the expansion of the air when rising has been disregarded.